The use of optical memory techniques with high-density and high-capacity optical disks is extending to a digital audio disk, video disk and data file, where, for example, a digital video disk with a density and capacity seven times or more that of a conventional optical recording medium has been developed.
In the optical memory techniques, a high-performance optical system technique in optical head device is required to conduct the recording and reproducing of information with a high reliability through a light beam converged to the order of micrometer.
An optical head device where optical system techniques are integrated has three basic functions, i.e., a convergent function to form a microspot at diffraction limit, a function to control the focusing of a microspot by its optical system and detect a pit signal, and a function to control the tracking of a microspot by its optical system.
These functions are achieved by using various combinations of optical system and photoelectric conversion detection system depending upon the object and use.
Next, a conventional optical head device will be explained in FIG. 1.
The optical head device is composed of a semiconductor laser 9, a diffraction grating 11, a beam splitter 12a, an objective lens 10 and a photodetector 13. A microspot is formed on an optical disk 8 by projecting a light beam and reflected light is used as an information reproducing signal.
In operation, light emitted from the semiconductor laser 9 is transmitted through the diffraction grating 11, forming three beams to detect a tracking error signal, then reflected by the beam splitter 12a, converged by the objective lens 10. Light reflected on the signal surface of the optical disk 8 is supplied through the objective lens 10 and beam splitter 12a to the light-receiving surface of the photodetector 13, converted into an electric signal, thereby obtaining an information signal.
Conventionally, the focusing error signal detection for controlling the microspot to follow the plane pitching of the optical disk 8 is conducted by again supplying light reflected on the optical disk 8 to the objective lens 10, then producing astigmatism at, e.g., the beam splitter 12a, supplying it to the photodetector 13. When the microspot projected onto the optical disk 8 is out of a focal position, an error signal according to the amount of the plane pitching of the optical disk 8 is obtained. Thus, a control signal for canceling the error signal can be applied to a lens actuator (not shown) to control the microspot to be on the focal position.
The tracking error signal detection for controlling the microspot to follow the eccentricity of the optical disk 8 is conducted by projecting three beams produced by the diffraction grating 11 onto the optical disk 8, setting the rotation position of the diffraction grating 11 so that .+-.1st-order diffracted lights are placed in order in the forward direction of tracking pits, detecting a difference between +1st- and -1st- diffracted light signals. When the microspot projected onto the optical disk 8 is out of a target track position, an error signal according to the amount of the eccentricity of the optical disk 8 is obtained. Thus, a control signal for canceling the error signal can be applied to the lens actuator to control the microspot to be on the target track position.
Next, another conventional optical head device with a collimator lens 14 will be explained in FIG. 2, wherein like parts are indicated by like reference numerals as used in FIG. 1.
When a signal is reproduced from the optical disk 8, light emitted from the semiconductor laser 9 is converted through the collimator lens 14 into collimated light. The collimated light is transmitted through the beam splitter 12a, then formed into a microbeam by the objective lens 10.
The microbeam is projected on the optical disk 8. When a microspot is projected on an information signal area so as to reproduce information recorded on the optical disk 8, the position control of the light beam against the plane pitching and eccentricity of the optical disk 8 is necessary.
Due to the plane pitching and eccentricity, the light beam converged to about 1 .mu.m may be out of the information signal area. The objective lens 10, which is held by a lens actuator (not shown) movable in the directions of two axes for focusing and tracking, controls precisely the converged spot to be on the information signal area.
In the above optical information reproducing devices, the increase in recording capacity has been desired. Thus, in such a device, a light spot projected on an optical disk needs to be miniaturized to achieve a higher-density recording.
The size of the microspot on the optical disk depends upon a wavelength .lambda. of the laser and a numerical aperture (NA) of the objective lens. Therefore, in the current techniques, the size reduction of the projected spot is achieved by decreasing the wavelength .lambda. and increasing NA.
When a microspot is produced, the wavefront aberration of an optical part in optical head device, such as a semiconductor laser, diffraction grating, beam splitter and objective lens, affects directly the deterioration in the size of a microspot.
Namely, the wavefront aberration of each optical part deteriorates image formation characteristics of the optical head device. If the wavefront aberration of each optical part is big, a microspot diameter which is determined by NA of objective lens and a wavelength of semiconductor laser cannot be produced.
According to Marechal's criterion, it does not become less than 80% of an ideal convergence intensity characteristic when the total wavefront aberration of optical parts in an optical head image-formation system is less than 0.07 .lambda..
"Micro-Optics Handbook", Vol.5, p.625 reports general wavefront aberrations for optical parts in conventional optical head devices, i.e., 0.013 .lambda. for semiconductor lasers, 0.025 .lambda. for collimator lens, 0.012 .lambda. for diffraction gratings, 0.015 .lambda. for beam splitters, 0.03 .lambda. for objective lens, 0.05 .lambda. for optical disks and 0.025 .lambda. as adjustment part, whereby the total wavefront aberration of an optical head device is given by: ##EQU1##
In high-density optical disks, laser light may fail to diffract entering into a pit since the pit size becomes smaller than a wavelength of light source. If it fails to diffract, an information signal recorded on the optical disk cannot be reproduced because the information signal is reproduced converting into an electrical signal by using the diffraction of light projected on the pit. Namely, in case of high-density optical disks, an allowable criterion of total wavefront aberration of an optical head device must be further severer than 0.072 .lambda.. Thus, an increase in wavefront aberration of optical parts will sensitively promote the deterioration of an information signal in high-density optical disks. Therefore, there occurs a problem that recording and reproducing may not be stably conducted.
Meanwhile, the numerical aperture (NA) of an objective lens in an optical head device becomes bigger so as to reproduce, particularly, a high-density optical disk. Therefore, a convergent beam on the recording surface of the optical disk becomes sensitive to a deterioration in aberration, and it is very difficult to obtain a desired convergent beam diameter. Namely, NA of objective lens has to be designed bigger to reproduce the high-density optical disk. In this case, a microspot diameter is designed at such a limit that the objective lens can diffract. Thus, even a little dispersion of wavefront aberration of the optical part causes a big variation in the shape of a convergent spot.
To this problem, several solutions have been conducted, for example, the performance test or production control of optical parts of an optical head device, such as an objective lens, is severely conducted. However, in the conventional solutions, there are problems that the optical head device produced is costly and that the production yield is lowered.